The heating devices used in food cooking or food warming, especially in portable appliances or utensils, use a variety of technologies such as pre-heated ceramic discs and external electrical coil heaters that heat water, candles, and chemical burners. Each of these has problems in uniform heat distribution. Furthermore, in some applications, such as portable food warmers, these methods of maintaining heat present problems in transporting the containers in which the food is held. Carbon fiber tow heaters solve the problems by serving as integral parts of the containers in which food is held. In recognition of the aforementioned problems an apparatus directed to the safe, reliable cooking and maintenance of food at the proper temperatures used in both transportation and at locations for serving is needed.
Utilizing carbon fiber technologies generally presents a problem in terminating the fiber tow, so that it can be reliably used to conduct current. Prior art connectors used in the creation of heating element products often limit the maximum operating temperatures before degradation, reliability, product life cycle and serviceability. U.S. Pat. No. 7,662,002 (Johnston) discloses an assembly for connecting a tow of axially elongated carbon fibers with a plurality of discrete contact portions, referred to as a tow into a metal “U” shaped trough with knurled ridges. Manufacturing this type of connector requires pressing down a top male die with ridges to squeeze the carbon fiber layers and then uses ultrasonic welding to fix the fibers to contact points. A pneumatically activated carriage mechanism applies pressure to the preassembled parts. The '002 processes uses a 1000 watt ultrasonic welder producing a 20 kHz frequency and a long weld time of 600 milliseconds at 60 joules of energy. Once the fibers are welded to the connector they become an integral part of the fiber tow preventing a complete substitution of the entire assembly in the event there is a malfunction in the field. In addition these connectors have other problems that limit their usability. One problem arises because the ultrasonic energy causes the carbon fibers to vibrate and some portion of them migrate beyond the sides of the polyester film, causing shorts to ground when voltage is applied. This technique diminishes the reliability of the carbon tow device. Furthermore, the method of manufacture utilizing ultrasonic welding also slows down the manufacturing of the assembly. Additionally ultrasonic welding of carbon fibers to metal is unreliable when the connector temperature exceeds a temperature of 400° F. For flat heating elements utilizing carbon fiber tows, contained in a polyester sheath, the temperatures cannot exceed a temperature of 350° F. before the connector itself and the polyester suffer permanent damage. As will be described below, a novel sheathing material utilizing in one instance a copper foil in combination with a copper braid to attached to the carbon fiber tow, allows the temperature of the carbon fiber tow to exceed a temperature of 700° F. and therefore any connector utilizing ultrasonic welding is unsuitable. In an alternate connecting device a metal clam shell connector as describe in the related U.S. Pat. No. 8,274,019 for a High temperature connector and method for manufacturing does allow the temperature of the carbon fiber tow to exceed a temperature of 700° F.